THE ENIGMA OF WHIPLASH
It's happened to many of us. You're in rush hour traffic and you are forced to bring your vehicle to an abrupt stop. You glance in your rear view mirror and think, "That guy's closing in on me pretty fast!" Next is the sound of "KABOM" as your vehicle is struck in the rear.
Rear end accidents such as this happen to thousands of people each year. Although they range from incidents of no visible contact to those of totaled vehicles, the net result is material damage accompanied by hundreds of millions of dollars in soft tissue injury or "whiplash" claims. In fact, in British Columbia in 1993, whiplash claims accounted for an estimated 70 percent of all injury claims, roughly $700 million worth, or over $200 per BC resident.
Whiplash is the commonly used term to describe the soft tissue strain injury which, according to the current medical literature, results from hyper-extension of the cervical portion of the spine. Most incidents begin in the same way. The rear vehicle approaches and then contacts the stationary vehicle ahead. As the bumpers engage, the front vehicle is shunted forward, in most cases only a short distance. As the front vehicle moves forward, the vehicle seatback presses against the occupant's back accelerating their body forward. The inertia of the occupant's head causes the head to lag behind the forward moving body. The head rotates rearward relative to the body, normally striking the headrest portion of the seatback. After the acceleration phase is over, the head and body then rebound slightly forward from the seatback, in some cases lightly touching the seatbelt restraint.
Researchers have proposed that cervical extension should be limited to less than 60 degrees to prevent whiplash injury. For lower speed impacts, this is normally accomplished by utilizing a properly adjusted headrest, where the top of the headrest is at least as high as the occupant's ear. They further indicated that as the level of cervical extension approaches the injury limit, the critical injury parameter is the equivalent moment or torque level, below which soft tissue strain would not be expected. This data was utilized along with vehicle impact test data to determine a level of rear impact, below which no injuries would be expected.
The final result is that soft tissue strain injury (whiplash) would not normally be expected for rear contacts with a magnitude of less than 8 km/h EBS (EBS = equivalent barrier speed, or the speed at which the vehicle strikes a rigid wall). Conveniently, this level of impact magnitude is close to the capacity threshold of a Canadian vehicle's bumper system. Therefore, if the impact was within the capacity of the bumper system, without secondary damage to the vehicle, injuries would not normally be expected. This has lead to the sometimes used concept of "No crash, No cash".
However, injury potential is dependent on a number of factors. Impact magnitude is the most critical, with "No crash" impacts ranging between a nudge contact at 2 km/h EBS, and an impact slightly exceeding the limit of the bumper system. For the occupant, the former represents a "slap on the back" while the latter exposes the occupant to forces and motions approaching the levels where injury would not be unexpected. Between the two extremes, the injury potential is influenced by headrest support, seat support, seated posture, head posture, predisposition to injury, physical conditioning, previous injuries, occupant sex and age.
An engineering analysis typically focuses on a determination of the impact magnitude and the level of headrest support utilized and available in the vehicle. Normally, both the striking and the struck vehicles are examined to document the areas of related contact. Special emphasis is given to determining signs of bumper movement or bumper overload. On the struck vehicle, measurements are taken to define the seat support and the utilized and available headrest support.
Using the examination information, the analysis involves determining the impact configuration of the vehicles, the magnitude of the contact experienced by the front vehicle, the expected induced occupant movements and resultant head torque levels, the utilized headrest support and the available headrest support. Combined, these allow an evaluation of the injury potential of the incident. The available headrest support can be assessed relative to the utilized headrest support, to determine if the occupant negligence could have contributed to an increase in injury potential during the contact.
Using the physical evidence of vehicle to vehicle impacts, along with the expected injury potential still leads us to the concept of "No crash, No cash". The enigma lies in the fact that in Canada, each year thousands of people claim whiplash injuries in low speed collisions where none would be expected. These amount to hundreds of millions of dollars in injury claims, most of which cannot be objectively diagnosed by existing medical technology. This makes it virtually impossible to separate legitimate and fraudulent claims. Once this problem is addressed, the claim amounts will surely be reduced, and the medical community will have a better understanding of the most efficient methods of preventing and treating these conditions. This will benefit all members of the driving community.
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